There are vacuum cleaners or devices known in the art which are capable of taking up liquids. An electric motor generates a vacuum to draw in and collect liquids and solutions in a recovery tank. There are also know devices which draw in dust and other solid particles, as well as water, in the recovery tank. These devices will hereinafter be collectively referred to a liquid vacuum cleaners. A typical liquid vacuum cleaner device comprises a container for holding a cleaning liquid or water, a pump to dispense and work the liquid on the surface to be cleaned, and a powerful suction device draws in the liquid and deposits it in a recovery tank.
A problem with such liquid vacuum cleaners is that once the water level of the tank rises above a certain level, there is a risk of overflowing from the tank and interfere with the proper operation of the vacuum cleaner. Consequently, level-controlled disconnection systems have been used to switch off the motor once a predetermined water level is reached. One known system employs a water sensor arranged at a predetermined height inside the tank which may consist of two separate current-carrying wires. When liquid is drawn into the tank, the two respective wires have resistance values differing in such a manner that this changed state can be detected and evaluated for switching off the motor.
Another system employs a float within the recovery tank. This float is connected with string that is connected to a micro switch. As the tank is filled, the float will rise and block the sucking orifice of the motor at the top of the tank. In doing so, the float pulls the string, and opens the micro switch to stop the motor. In this system, incomplete blocking of the orifice may give rise to overflowing. Water may also be sucked into the motor, thereby shortening the life of the motor. Even where the blocking is complete, the motor is still rotating, which also shortens the motor's life, as well as waste energy.
Another problem with vacuum motors, especially ac motor, arises with the shut-off switching of the motor or commutation of the thyristor. Internal switching by a thyristor such as a bidirectional triods thyristor, or triac, may arise from its dv/dt characteristic. Commutation is more severe with inductive loads (e.g., motors) than with resistive loads because of the phase lag between voltage and current. Once the triac is switched, it only has a brief interval during which to recover. At this time the current of the power handling triac falls below the holding current, the triac ceases to conduct. If the voltage across the triac rises to rapidly, the triac will resume conduction and loss of control is the result.
In-rush current which occurs during the initial start up or switching on of the power supply can have a deleterious effect on electrical devices. The initial in-rush current forces the motor jump to its operating speed immediately and abruptly. This places a great deal of stress and strain on the vacuum motor, thereby shortening the useful operating life of the motor. In addition, high in-rush current during the switching on procedure may also lead to a decrease in the useful life of the brushes and power line switch.